Schedule delayed rent for the tenant with the given external ID.

Requirements:

• The user must be approved (active membership).
• The current mocked date must be in the rent window.
  Use POST /simulation/tenants/{external_id}/date to set it if needed.

⚠️ Returns 400 if any of the above requirements are not met.

What this does:

• Schedules a delayed rent payment using default installment values derived from the tenant's portal scan.

After calling this endpoint, use POST /simulation/delayed_rent/execute/{external_id} to trigger execution.